RE: Why the load index of single tire is higher than double tire's?

I will post a concise citation when I find one. The continental sales training material over simplifies the subject . The reason is load on the individual tires and the factors that affect that.
1 is indeed crown , if the road surface is at angle to the tires the inner tire will have a greater percentage of the load .
2. even 2 new identical tires will not be exactly the same dimensions as they wear differences will increase , the larger tire will carry more load.
3. camber of the axle and how concentric the wheel mounting is will affect how much load is placed on each tire.

The 9 % is a margin of safety , I did not remember that exact number ,but I don't remember many things.
Ed

Since the later 1980s, manufacturing companies have seen over 3 percent in labor output year on year, with most of this productivty enhancement occuring because of the optimization of repetitive tasks. Download Now